Handover techniques may include a mobile station initiative type handover and a network initiative type handover widely adopted in cellar systems such as PDC and IMT-2000.
The network initiative type handover is a handover scheme in which a mobile station side is caused to measure the reception level and the reception quality of neighboring cells and to report information regarding switch destination candidate cells as a control signal to the network side. The network initiative type handover is based on a so-called “mobile station assist type handover”.
In response to receipt of the control signal, the network side determines a switch destination cell in consideration of information possessed in the network side such as the status of radio resource utilization of the switch destination candidate cells. Also, after a switch ready instruction is issued to a base station covering the switch destination cell, the network side informs the mobile station of the switch destination cell and performs a handover operation. This handover is generally used for cell switching of a communication channel required to maintain the communication quality.
The network initiative type handover is characterized in that the mobile station side selects switch destination candidates whereas the network side determines the final switch destination and in that the cell switching in the mobile station side can be carried out at the same timing as the switching of a transmission route in the network side. As a result, more reliable and faster handover toward the optimal cell can be achieved.
However, the network initiative type handover has some disadvantages in that numerous control steps may be required and that some additional control statuses such as a signal waiting status may have to be provided. In particular, if a mobile station has not been successfully handed over to a designated switch destination cell, some complicated operations may have to be carried out to address this case. The network side performs the route switching for user data independent of the fact that the mobile station side successfully has captured the radio wave for the handover destination cell. Thus, if the handover fails, their statuses do not coincide with each other, and some operation for reverting the above operations must be conducted.
Here, if a proper area configuration is designed, the handover failing has a lower probability. However, the handover may still fail with a certain probability due to some unavoidable difficulties such as temporary out-of-service in an area edge and a tunnel. Thus, such situations have to be taken into consideration.
On the other hand, the mobile station initiative type handover is a handover scheme in which a mobile station autonomously determines the switch destination cell based on the reception level and/or the reception quality of several adjacent cells and performs a handover operation at an arbitrary timing.
According to the mobile station initiative type handover, some control steps can be combined for the purpose of shorter radio synchronization times in such a manner that switch destination candidate cells are reported to the network side in advance and information regarding a channel for the switch destination cell is received from the network side. Unlike the network initiative type handover, however, control steps such as reservation of radio resources and setting of the switching timing are not required in the network side.
The mobile station type initiative type handover is mainly used in cell switching under the communication status in a common channel or in reconnection in the case of failure of the network initiative type handover.
The mobile station initiative type handover is characterized in that since a transmission route is changed after completion of both the autonomous cell switching in the mobile station side and the cell switching in the network side, the overall control can be simplified. Even if the mobile station side cannot successfully capture the radio wave from the initially determined cell, the control of the network side can be initiated without influence of the operation of the network side after successful retry in any cell.
However, since the mobile station autonomously performs the cell switching, the handover cannot be fulfilled in consideration of information possessed in the network side such as the status of radio resource utilization. In particular, the mobile station initiative type handover may not be suitable for voice service and video telephony service in which radio resources are required to maintain a certain communication quality. In addition, the route switching of user data is performed after completion of the handover is confirmed through a control signal, resulting in temporary increase in transmission delay.